Natural gas is a mixture of gaseous hydrocarbons and non-hydrocarbon impurities and contaminants. Removal of carbon dioxide and acidic sulfide contaminants (e.g., CO2, H2S, RSH, CS2, COS, SO2, etc.) to meet quality and regulatory requirements in natural gas that is fed into distribution pipelines is a major industrial issue. In addition, increasing concerns of global warming from CO2 and other emissions has prompted significant investments into methods of capturing such contaminants. Moreover, such contaminants are often corrosive and impair the caloric value of the gas.
Aqueous solutions of commonly available commodity alkanolamines are generally used as scrubbing solutions (chemical absorbents) in gas processing. The purpose of these scrubbing systems is to remove acidic contaminants from the raw gas stream. As energy sources are being depleted and environmental restrictions are tightening, the economic use of the “bottom of the barrel” in gasification processes is increasing. There are many new projects being sanctioned, most of which would need acid gas clean-up to remove contaminants before processing. Removing CO2 from flue gases is important for a variety of reasons, such as reducing greenhouse gases and providing a concentrated CO2 feed to the secondary CO2 and enhanced oil recovery markets.
Weak organic bases, such as monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA) comprise many of the typical alkanolamine solvents known in the art. MDEA is known to have advantages for CO2 removal and other acid gas contaminants in high-pressure gas streams. The amount of energy required to regenerate the MDEA is low because it is a relatively weak base and therefore the chemical bond formed during the reaction with CO2 is weaker than with other commonly used alkanolamines. A secondary benefit lies in the nature of the chemical bond formed during absorption. As a tertiary alkanolamine, MDEA reacts with CO2 to form a bicarbonate ion rather than a carbamate, which results in a reaction ratio MDEA to CO2 of 1:1. In contrast, other commonly used primary and secondary alkanolamines preferentially form a carbamate and require a reaction ratio of 2:1. The reaction between CO2 and tertiary alkanolamines (e.g., MDEA) is typically of a greater efficiency than between CO2 and other commonly used primary and secondary alkanolamines. These combined benefits result in a process of greater efficiency and capacity than is possible with commercial primary and secondary alkanolamines such as MEA and DEA.
A disadvantage of using tertiary alkanolamines is that CO2 is indirectly absorbed, resulting in a weak driving force and slow rate of reaction compared to other commercial alkanolamines. In high-pressure gas contacting systems the effect of the weak driving force is minimized due to the higher fraction of CO2 in the liquid resulting from the high CO2 partial pressure in the gas above it. When gasses are contacted at low pressure, the driving force is weak as the partial pressure of CO2 is also weak. Thus, there is no beneficial effect of pressure, and the CO2 equilibrium established between the gas and liquid is low. Tertiary alkanolamines are not normally used in low-pressure applications because of their low equilibrium loading. Other more commonly used primary and secondary amines such as MEA and DEA, which are stronger bases, are used in these applications due to their higher driving force and increased rate of reaction with CO2. In these low-pressure situations, the disadvantage of the inefficient carbamate reaction is outweighed by the greater equilibrium liquid distribution achieved.
In an effort to increase the capacity of MDEA for CO2 at low partial pressure, a number of improvements to the basic MDEA process have been developed. These improvements typically involve the addition of small amounts of primary or secondary amines to the MDEA solution (as described in U.S. Pat. Nos. 5,209,914 and 5,366,709 and PCT Application No. WO 03/013699). The resulting mixtures are commonly described as formulated or blended MDEA with additives referred to as “catalysts,” “absorption accelerators,” or “activators” (e.g., U.S. Pat. No. 6,740,230). These additives generally function by increasing the rate of CO2 absorption into the MDEA blend solution at low CO2 partial pressure thereby increasing the fraction of CO2 in the liquid as compared to the MDEA solution alone.
Although effective in the removal of CO2 as described, the commercial application of known formulated solvents has less than ideal operating characteristics. Some of the additives used for formulating have limited solubility in MDEA, which reduces their effectiveness, and their lower boiling points in turn create difficulties in maintaining their concentration. Moreover, the reaction products of the additives with CO2 are also problematic. As they are stronger organic bases than MDEA these blends have a tendency to require more energy for regeneration and have limited solubility. Such characteristics limit their effectiveness and efficiency of the overall process if their concentration exceeds approximately 20%.
There thus exists an industrial need for improved compositions and methods for recovering acidic contaminants from both high and low pressure systems. A particular need exists for products having the benefits of both low-pressure equilibrium capacity of primary or secondary amines and the efficiency of tertiary amines within a single compound of reduced volatility.